Ionic liquids from renewable biomaterials: Synthesis, characterization and application in the pretreatment of biomass

Article abstract of DOI:10.1039/c2gc16128a

A series of room temperature ionic liquids (ILs), in which cholinium acts as the cation and amino acids as the anions, were prepared via a simple and green chemical route, and characterized. Most of the ILs dissolved lignin efficiently and selectively (with solubilities of 140-220 mg of lignin per g of IL). The solubility of xylan in these ILs (which ranged from <1 to 85 mg g^-1) depended on the nature of the anion, while cellulose was scarcely soluble (<5 mg g^-1). In addition, enzymatic hydrolysis of microcrystalline cellulose and rice straw was enhanced significantly after pretreatment using the IL [Ch][Gly].

Full text of DOI:10.1039/c2gc16128a

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COMMUNICATION
Ionic liquids from renewable biomaterials: synthesis, characterization and
application in the pretreatment of biomass†
Qiu-Ping Liu,‡a Xue-Dan Hou,‡a Ning Li*b and Min-Hua Zong*b
Received 9th September 2011, Accepted 21st November 2011
DOI: 10.1039/c2gc16128a
A series of room temperature ionic liquids (ILs), in which
cholinium acts as the cation and amino acids as the anions,
were prepared via a simple and green chemical route, and
characterized. Most of the ILs dissolved lignin efficiently
and selectively (with solubilities of 140–220 mg of lignin per
g of IL). The solubility of xylan in these ILs (which ranged
from <1 to 85 mg g-1) depended on the nature of the anion,
while cellulose was scarcely soluble (<5 mg g-1). In addition,
enzymatic hydrolysis of microcrystalline cellulose and rice
straw was enhanced significantly after pretreatment using
the IL [Ch][Gly].
stable chiral centers and reasonable cost.13 A group of amino
acid-based ILs, in which amino acids act as the cations or anions,
have been reported since 2005.14–17 However, less attention has
been paid to ILs with cholinium as the cation and amino acids
as the anions ([Ch][AA]) and reported applications of such
ILs have focused solely on their activities as catalysts.8,9 It is
worth noting that to date the physicochemical properties of such
ILs remain unknown. In the present work, eighteen [Ch][AA]
ILs were synthesized from choline hydroxide and amino acids
via simple neutralization reactions, and their physicochemical
properties were characterized. In addition, their application in
the pretreatment of lignocellulose was examined.
Room temperature ionic liquids (RTILs) have been attracting
increasing attention as electrolyte materials, catalysts and sol-
vents for synthesis, catalysis and extraction etc., due to their
excellent properties such as low vapor pressure, nonflammability,
high thermal and chemical stability, and outstanding ability to
dissolve a wide range of compounds.1–3 Among a large number
of ILs that have been synthesized and characterized, traditional
ILs that consist of imidazolium or pyridinium cations and
halide-containing anions have been demonstrated to be highly
toxic and poorly biodegradable.4 In addition, the feedstocks
used to produce these ILs are of nonrenewable petroleum
origin. Therefore in recent years, production of non-toxic and
environmentally benign ILs from renewable materials has been
proposed.5,6 Choline, an essential nutrient, is known to be
non-toxic and biodegradable.7 Recently, a group of cholinium-
based ILs have been synthesized and characterized.8–11 These
ILs showed excellent biodegradability6 and low toxicity to fila-
mentous fungi6 and the freshwater crustacean Daphnia magna.12
Amino acids, as one of the most abundant sources and chiral
pools in nature, are excellent feedstocks for the synthesis of ILs,
especially the ILs used in catalysis and synthesis, owing to their
Eighteen [Ch][AA] ILs were synthesized via neutralization
reactions with the yields of more than 95%. The water contents
were less than 0.08 wt% in all these ILs. The structures of the
ILs were confirmed by 1H NMR, elemental analysis, and FT-IR
(available as supplementary materials†). Based on differential
scanning calorimetry (DSC), no melting points were observed
in the temperature range scanned for all the ILs (Fig. 1a). All
the [Ch][AA] ILs were liquids at room temperature. As shown
in Table 1, the glass transition temperatures (Tg) ranged from
-74 to -10 ◦C, and the Tg varied with the structures of the
anions. Generally, an increase in the number of carbon atoms
in the side chain of amino acid resulted in an increase of Tg
(Table 1, entries 1, 2, 6 and 7). However, [Ch][Val] was an
exception, having the lowest Tg (-74 ◦C) among all the ILs
(Table 1, entry 5). Moreover, the introduction of an additional
hydroxyl or carboxylic acid group led to increasing Tg values
(Table 1, entries 3, 4, 12 and 13), possibly due to strong hydrogen
bond interactions.5,15 For example, [Ch][Asp] and [Ch][Glu]
exhibited relatively high Tg (-22 and -18 ◦C, respectively). The
effect of phenyl ring on the Tg was negligible (Table 1, entry
9), which might be attributed to larger electron delocalization
counteracting the effect of increased side chain size. For the
ILs containing basic amino acids as the anions, the higher Tg
might stem from hydrogen bonds involving the extra amino or
imidazole group (Table 1, entries 16–18).
aCollege of Biosciences and Bioengineering, South China University of
Technology, Guangzhou, 510006, China
bState Key Laboratory of Pulp and Paper Engineering, College of Light
Industry and Food Sciences, South China University of Technology,
Guangzhou, 510640, China.
The thermal gravimetric analysis (TGA) curves of several rep-
resentative [Ch][AA] are shown in Fig. 1b. The decomposition
temperatures (Td) were calculated from the intersection of the
baseline and the tangent line in the TGA curves, as described
previously.18 As shown in Table 1, the Td values were in the
range of 150–203 ◦C, which were comparable to those of amino
E-mail: lining@scut.edu.cn, btmhzong@scut.edu.cn; Fax: +86 20 2223
6669; Tel: 86 20 8711 1452
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c2gc16128a
‡ Equally contributing authors.
304 | Green Chem., 2012, 14, 304–307
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Table 1 Properties of the [Ch][AA] ILs
Entry ILs
Tg/◦Ca Td/◦Cb Viscosity/mPa·sc [a]2D0d
1
2
3
4
5
6
7
8
[Ch][Gly]
-61
-56
-55
-39
-74
-47
-47
-61
-60
-12
-44
-22
-18
-14
-40
-48
-40
-10
150
159
182
172
177
175
175
178
160
174
163
202
202
187
203
165
171
163
121
163
402
454
372
476
480
330
520
5640
500
2060
2308
1903
2589
460
—
[Ch][Ala]
[Ch][Ser]
[Ch][Thr]
[Ch][Val]
[Ch][Leu]
[Ch][Ile]
[Ch][Met]
[Ch][Phe]
[Ch][Trp]
[Ch][Pro]
[Ch][Asp]
[Ch][Glu]
[Ch][Asn]
[Ch][Gln]
[Ch][Lys]
[Ch][His]
[Ch][Arg]
+1.00
-4.68
-1.33
+8.39
-1.21
+4.09
-2.33
-9.62
-13.99
-65.49
-31.60
-10.19
-15.62
-5.25
+4.50
-9.74
+8.33
9
10
11
12
13
14
15
16
17
18
980
1002
a Glass transition temperatures (Tg) were determined by DSC with a
◦
heating rate of 10 C min-1, after cooling samples to -70 ◦C under
nitrogen. b Decomposition temperatures (Td) were measured by TGA
with a heating rate of 5 ◦C min-1 under nitrogen. c At 25 ◦C. d Solution
in CH3OH (c = 2).
In addition to the molecular size of the anion, the introduction
of extra carboxylic acid or amide group substantially increased
the viscosity of the ILs (Table 1, entries 12–15), possibly owing to
strong hydrogen bond interactions. For instance, high viscosities
(1903–2589 mPa·s) were recorded for [Ch][Asp], [Ch][Glu] and
their amide analogs. The ILs with basic amino acids as the
anions displayed relatively high viscosities (980–1002 mPa·s),
with the exception of [Ch][Lys] (460 mPa·s). Surprisingly, among
the eighteen [Ch][AA] ILs, [Ch][Trp] was the most viscous (5640
mPa·s). The rheological behaviors of the ILs including shear
rate and temperature dependence of the viscosity were studied
(Fig. 1S, available as supplementary materials†). The apparent
viscosity of highly viscous ILs was substantially reduced with
the increase of shear rate and temperature, whereas the apparent
viscosity of less viscous ILs like [Ch][Gly] was independent on
shear rate and temperature. High viscosity ILs, such as [Ch][Trp],
and [Ch][Asp], showed shear thinning property, suggesting the
formation of cross-linked hydrogen bond network in the ILs
or aggregation of ions with different forms, such as anion
stacking and tail aggregation.23 The increase of shear rate
and temperature disrupted the ions aggregation and cross-
linked network, thus leading to the reduction of the viscosity.24
In addition, larger anion size and stronger intermolecular
forces such as van der Waals, hydrogen bond, and p-stacking
interactions also might contribute to the high viscosity.
The optical rotations, [a]2D0, were measured in methanol
solutions (Table 1). The optical rotations of some ILs were
different from those of amino acids they contained. A similar
phenomenon was reported previously by Allen et al.25
Cellulose and lignin are, respectively, the first and second
most abundant renewable organic polymers on earth; and
combined with hemicellulose, they constitute the structural
components of plants.26 Recently, cellulose and hemicellulose
have received increasing interest as feedstocks for the production
of biofuel and many platform chemicals, to combat the gradual
Fig. 1 DSC and TGA curves for several representative [Ch][AA] ILs.
acid-based ILs reported previously.15 Nevertheless, they are
thermally stable enough to meet the criterion of the minimum
◦
Td of 100 C for chiral ILs proposed by Wasserscheid.19 Also,
the Td showed a clear dependence on the anion structures. Of
the ILs tested, [Ch][Gly] gave the lowest Td (150 ◦C). Similarly,
elongation of the side chain and introduction of a hydroxyl or
carboxylic acid group resulted in higher Td values, whilst the
introduction of a phenyl ring exerted no effect on the Td (Table
1, entry 11) since [Ch][Phe] has almost the same Td as [Ch][Ala]
(160 vs. 159 ◦C). The Td of [Ch][Pro], [Ch][Phe], [Ch][Gly] and
[Ch][Thr] are consistent with the previous results, but [Ch][His]
synthesized in the present work◦ is much more stable than that
reported by Moriel (171 vs. 128 C).8,9 And the other basic amino
acid-containing ILs have comparable Td (163–165 ◦C).
Viscosity is one of the most important factors affecting the
applications of ILs, especially as the solvents for catalysis and
extraction etc. As a result, great efforts have been made to
design and synthesize ILs with low viscosity.20–22 The viscosities
of the ILs synthesized◦ during this work are in the range of
121–5640 mPa·s at 25 C (Table 1). An increase in the size of
the anion generally resulted in a higher viscosity, which might
be ascribed to stronger van der Waals and/or hydrogen bond
interactions.10 For example, [Ch][Gly], with the anion being the
simplest amino acid, displayed the lowest viscosity (121 mPa·s).
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Table 2 The solubility of lignin, xylan and cellulose in [Ch][AA] ILs at
90 ◦
of lignin and low solubility of the other biopolymers mean that
[Ch][AA] ILs are attractive solvents for highly efficient and
selective extraction of lignin from lignocellulose, which could
be used to allow total utilization of this biomass.
C
Lignin
Xylan
Cellulose pH
Entry
ILs
(mg g-1)
(mg g-1)
(mg g-1)
(5 mM)
The application potential of the [Ch][AA] ILs was demon-
strated by the following enzymatic hydrolysis experiments.
Initially, microcrystalline cellulose and rice straw were pretreated
using [Ch][Gly], a low viscosity IL, at 90 ◦C under N2 for
24 h. The remaining solid material was washed, lyophilized
and subjected to enzymatic hydrolysis. As revealed by Fig. 2,
the enzymatic hydrolysis efficiencies of both microcrystalline
cellulose and rice straw were significantly improved after being
pretreated by the IL. For example, the concentration of glucose
released upon enzymatic hydrolysis increased from 1.37 to
2.20 g L-1 after microcrystalline cellulose was pretreated with
[Ch][Gly]. The reduction of the cellulose crystallinity index
upon pretreatment might contribute to the higher enzymatic
hydrolysis efficiency.36 Rice straw without pretreatment was
found to be highly resistant to enzymatic hydrolysis, with
the concentration of glucose being only 0.31 g L-1 in the
hydrolysate. Interestingly, the pretreated rice straw could be
hydrolyzed readily by the cellulase. The concentration of glucose
reached 2.05 g L-1, which was approximately 7-fold higher
than that without pretreatment. This improvement in enzymatic
hydrolysis after pretreatment with the IL might mainly stem
from the selective removal of lignin, a biopolymer with high
resistance to biodegradation, from rice straw.32
1
2
3
4
5
6
7
8
[Ch][Gly]
[Ch][Ala]
[Ch][Ser]
[Ch][Thr]
[Ch][Val]
[Ch][Leu]
[Ch][Ile]
[Ch][Met]
[Ch][Phe]
[Ch][Trp]
[Ch][Pro]
[Ch][Asp]
[Ch][Glu]
[Ch][Asn]
[Ch][Gln]
[Ch][Lys]
[Ch][His]
[Ch][Arg]
220
180
170
160
70
150
170
150
140
90
170
<10
26
16
50
76
77
70
85
15
40
40
75
65
10
75
<1
<1
5
<5
<5
<5
<5
<5
<5
<5
<5
<5
<5
<5
<1
<1
<5
<5
<5
<5
<5
10.3
10.2
9.8
9.8
10.3
10.2
10.3
10.1
9.7
10.2
10.7
6.8
6.7
9.5
8.0
10.4
10.0
11.3
9
10
11
12
13
14
15
16
17
18
5
140
140
110
65
35
25
exhaustion of the conventional fossil resources.27,28 Besides,
lignin can be used as the starting material for preparation
of polyurethanes, polyesters and carbon fibers.29,30 Selective
extraction of individual components from biomaterials or wastes
is of particular interest.31–33 RTILs have emerged as a new and
practicable alternative to traditional media for separation and
extraction.34,35 Therefore, the [Ch][AA] ILs were evaluated for
their ability to dissolve lignin, xylan and cellulose (Table 2).
As shown in Table 2, lignin dissolved well in most of the
[Ch][AA] ILs at 90 ◦C, with the solubility being more than
140 mg g-1. In addition, there was a clear dependence of the
solubility of lignin on the alkalinity of the ILs, which could be
accounted for by the fact that lignin is a biomaterial known
to dissolve in alkali. For example, the solubility of lignin was
much higher in [Ch][Gly] than that in [Ch][Asp] (220 vs. <10
mg g-1), and at 5 mM, the corresponding pH values were
10.3 and 6.8 for [Ch][Gly] and [Ch][Asp], respectively. The IL
[Ch][Val] was an exceptional case, in which a low solubility
of lignin (70 mg g-1) was observed. Besides, the viscosity of
ILs also exerted an influence on the dissolution of lignin.
For example, poor dissolution of lignin was observed in the
highly viscous [Ch][Trp], although the pH of a 5 mM aqueous
solution of this IL was as high as 10.2. The lignin solubilities
in some of the ILs were also determined at room temperature
(Table 1S, available as supplementary materials†). Compared
to the solubilities of lignin in the ILs at 90 ◦C, solubilities
measured at room temperature were much lower (<115 mg
g-1 in all cases). In addition, the viscosity of the ILs seems
to be more significant than other factors in determining the
room-temperature solubility of lignin, because ILs are highly
viscous under these conditions. The dissolution of xylan was
much poorer in these ILs compared to lignin. The saturation
concentration of xylan ranged from 10 to 85 mg g-1 at 90 ◦C in all
the ILs except for [Ch][Asp], [Ch][Glu], [Ch][Asn] and [Ch][Gln]
(Table 2). Similar to lignin, the xylan solubility was affected by
the alkalinity of ILs as well as the viscosity. However, cellulose
showed little and even no solubility in all the ILs (<5 mg g-1),
independent on the anion that was present. The high solubility
Fig. 2 The effect of pretreatment with [Ch][Gly] on enzymatic hydrol-
ysis of cellulose.
Conditions for the hydrolysis reaction: 20 mg rice
straw/microcrystalline cellulose, 7 mL citrate buffer (50 mM,
pH 4.8), 35 U mL-1 cellulase from Trichoderma reesei, 50 ◦C,
200 rpm
In conclusion, we have successfully prepared a series of
new ILs from renewable biomaterials by simple neutralization
reactions. Due to their excellent physicochemical properties,
these [Ch][AA] ILs appear to be promising solvents for the
selective extraction of lignin from lignocellulose. The use of
these ILs may create new opportunities for the utilization of
lignocellulose. In addition, these [Ch][AA] ILs are expected to
have low or negligible toxicity and to be highly biodegradable,
306 | Green Chem., 2012, 14, 304–307
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since they are synthesized from environmentally friendly starting
materials. Studies to investigate these properties are ongoing in
our laboratory.
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This study was supported by the National Natural Science
Foundation of China (20876059, 20906032 and 21072065), and
the Major State Basic Research Development Program ‘973’
(2010CB732201). We are grateful to Prof. Thomas J. Smith
in Sheffield Hallam University for the helpful suggestions and
help in language improvement, and to Prof. Jinzhu Chen in
Guangzhou Institute of Energy Conversion for the help in NMR
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